1. Field of the Invention
The present invention relates to a piezoelectric ceramic and a method for producing a piezoelectric ceramic element and, more particularly, to a method for producing a monolithic piezoelectric ceramic element.
2. Background Art
Examples of conventional piezoelectric ceramic materials containing Pb, Zr, Ti, Cr, Nb and O include a ternary ceramic material represented by Pb(CrNb)O.sub.3 --PbZrO.sub.3 --PbTiO.sub.3 and a material comprising the ternary ceramic material and small amounts of a variety of additives. As disclosed in Japanese Patent Publication (kokoku) No. 51-28358 and Japanese Patent Application Laid-Open (kokai) No. 8-34667, such piezoelectric materials have excellent piezoelectric characteristics and processability, and can be produced on a large scale. Therefore, such materials are widely employed in the production of a piezoelectric element such as an actuator, a filter, a transducer or a sensor.
In recent years, there have been developed a variety of piezoelectric devices employing a monolithic piezoelectric element in which alternating layers of piezoelectric ceramic and internal electrodes are laminated. Although such a monolithic piezoelectric element is small and can advantageously generate a large displacement by the application of a small electric field, internal electrode layers and green sheets of a piezoelectric ceramic layer must be co-fired for the production of the piezoelectric element. Since the sintering temperature is 1200.degree. C. or more, an internal electrode material which can be co-fired with such a piezoelectric material is limited to an expensive material such as Pt. In order to lower the sintering temperature, PbO in an excessive amount or a glass component such as SiO.sub.2 has been added as a sintering aid. However, such a conventional method for producing a piezoelectric ceramic element has drawbacks.
Briefly, when the firing temperature to sinter a piezoelectric material is 1200.degree. C. or higher, evaporation of Pb present in the piezoelectric material increases drastically. In this case, stable piezoelectric characteristics cannot be realized.
In addition, when a glass component such as SiO.sub.2 is added as a sintering aid in order to co-sinter a piezoelectric material and internal electrode layers at 1200.degree. C. or lower, the sintering temperature can be lowered by about 50.degree. C. but the piezoelectric characteristics of the sintered product deteriorate. Particularly, the thus-obtained product cannot be employed in a piezoelectric device which must have large values of parameters of piezoelectric characteristics, such as an actuator.